Dual mode ramjet and scramjet engines can be used to propel an air vehicle at hypersonic speeds exceeding mach 5. For rocket-boosted dual mode ramjet and scramjet engine powered vehicles, an ignition system is required to deliver an initial energy source to induce combustion when operational conditions are not conducive to auto-ignition of the fuel/air mixture at take over condition. Additionally, a separate fuel source is typically needed for hydrocarbon-fueled dual mode ramjet and scramjet engines during “cold-start” when the main endothermic fuel is not at a temperature that can sustain combustion due to the fact that the engine flow path is not yet heated. Transitioning from the cold-start fuel to the primary endothermic fuel presents challenges to the design of propulsion systems, as the cold-start fuel and the main fuel each have different attributes in regard to ignition delay time, blowout, and heat release profile. The differences in these attributes result in unique engine/vehicle operational characteristics depending on whether the cold-start fuel or main fuel is being used, and vehicle control systems should be capable of addressing the transition from cold-start fuel to the main fuel.
Presently, dual mode ramjet and scramjet engine designs rely on a single-use pyrotechnic device, a plasma torch, or a spark igniter to generate the elevated temperatures and pool of radicals that are needed to ignite the fuel and air mixture. The single-use pyrotechnic device offers a single opportunity to start the engine. This can be problematic, as it may be difficult to restart the engine should the engine experience blowout or unstart during operation once the single-use pyrotechnic has been exhausted. The plasma torch and spark ignition systems, although theoretically capable of offering multiple restarts, undesirably require large electrical energy input to be effective.
Although multiple restart systems for dual mode ramjet and scramjet engines have been contemplated utilizing multiple single-use pyrotechnic igniters or plasma torches, both of these arrangements are undesirable for several reasons. Namely, the multiple single-use igniters and plasma torches undesirably add complexity, weight, cost, and power requirements to a vehicle while also requiring a separate cold-start system. Therefore, what is needed is a system that provides multiple restart capabilities to dual mode ramjet and scramjet engines while eliminating a need for a separate cold-start fuel system and also addressing the aforementioned transition challenges.